Abrasive sealing systems are currently used in gas turbines to reduce the blade tip gas leakage, which can improve turbine efficiency. In the turbine, the early stages of the high pressure turbine section are generally comprised of nickel-based superalloy blades that rotate at high speed. These blades are designed such that their respective tips are situated in close proximity to a stationary seal ring. As a result, a narrow gap exists between the tips and the seal ring. The purpose of the narrow gap is to minimize gas leakage and to allow the pressure of the air to do work against the turbine blades, thereby causing the blades to rotate. A narrow gap leads to higher engine efficiency and greater power output. However, a narrow gap also increases the risk of “tip rub”, defined as the tip contacting the seal and thereby causing excessive wear on the seal and the components.
The tips of the blades can be applied with an abrasive coating to improve the seal between blade tips and stationary ring. The abrasive coating comprises abrasive particles embedded in a metallic matrix, as described in FIG. 1. Cubic Boron Nitride (CBN) is a typical abrasive material used for such purpose. The metallic matrix holds or anchors the abrasive particles and maintains attachment of the particles to the turbine tip during service. To work effectively, the metallic matrix must have high-temperature strength to resist any deformation during service; environmental resistance to prevent material loss from high-temperature oxidation and corrosion; and chemical compatibility with the tips of the blades to ensure absence of formation of brittle phases, which are known as topologically close-packed phases (“TCP's”).
Conventional metallic matrix materials have typically utilized MCrAlY, where M is defined as nickel, cobalt, or nickel and cobalt in any combination. MCrAlY coatings are known for their environmental resistance. The MCrAlY coatings primarily consist of a gamma-nickel phase and a beta-nickel aluminide phase. Although MCrAlY coatings have excellent environment resistance, the MCrAlY coatings exhibit insufficient strength to resist the deformation at elevated service temperatures, particularly at 800° C. and higher. Another problem with conventional MCrAlY coatings is interdiffusion of these coatings with advanced superalloy substrates which causes phase instability at the interface and degrades the mechanical performance of the superalloy substrates. Thus, MCrAlY coatings tend to have a limited lifetime during service temperatures of 800° C. and higher. The term “service temperature” as used herein and throughout the specification refers to the range of operating temperatures a particular coated component is exposed to when in commercial usage. The service temperature serves as an indicator of the thermal stability of the coating.
Other metallic matrix materials have also shown limited operational lifetime at higher service temperatures. In view of the drawbacks, there is a need to increase the high-temperature strength of the metallic matrix for abrasive coatings while substantially maintaining the resistance to high-temperature oxidation. This is a need to improve performance of stand-alone coatings and overlay coatings requiring higher strength at their operating temperatures.